CDMA location

ABSTRACT

The invention determines the geographic location of a subscriber unit within a CDMA communication system. At least one base station transmits a spread spectrum signal with a chip code sequence unique to that base station. A subscriber unit receives the base station signal and transmits a spread spectrum signal with a unique chip code sequence time synchronized with the chip code sequence of the received base station signal. The base station receives the subscriber unit signal and compares the chip code sequence of the received subscriber unit signal with the chip code sequence signal transmitted by the base station to determine the location of the subscriber unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to spread spectrum code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a system and method that determines the geographic location of a subscriber unit within a CDMA communication system.

2. Description of the Prior Art

Wireless systems capable of locating a subscriber are presently known in the art. One wireless technique uses the global positioning system (GPS). In GPS, the communication handset receives data transmitted continuously from the 24 NAVSTAR satellites. Each satellite transmits data indicating the satellite's identity, the location of the satellite and the time the message was sent. The handset compares the time each signal was received with the time it was sent to determine the distance to each satellite. Using the determined distances between the satellites and the handset along with the location of each satellite, the handset can triangulate its location and provide the information to a communication base station. However, the incorporation of a GPS within a subscriber unit increases its cost.

Another subscriber location technique is disclosed in U.S. Pat. No. 5,732,354. A mobile telephone using time division multiple access (TDMA) as the air interface is located within a plurality of base stations. The mobile telephone measures the received signal strength from each of the base stations and transmits each strength to each respective base station. At a mobile switching center, the received signal strengths from the base stations are compared and processed. The result yields the distance between the mobile telephone and each base station. From these distances, the location of the mobile telephone is calculated.

Wireless communication systems using spread spectrum modulation techniques are increasing in popularity. In code division multiple access (CDMA) systems, data is transmitted using a wide bandwidth (spread spectrum) by modulating the data with a pseudo random chip code sequence. The advantage gained is that CDMA systems are more resistant to signal distortion and interfering frequencies in the transmission path than communication systems using the more common time division multiple access (TDMA) or frequency division multiple access (FDMA) techniques.

There exists a need for an accurate mobile subscriber unit location system that uses data already available in an existing CDMA communication system.

SUMMARY OF THE INVENTION

The invention determines the geographic location of a subscriber unit within a CDMA communication system. At least one base station transmits a spread spectrum signal with a chip code sequence unique to that base station. A subscriber unit receives the base station signal and transmits a spread spectrum signal with a unique chip code sequence time synchronized with the chip code sequence of the received base station signal. The base station receives the subscriber unit signal and compares the chip code sequence of the received subscriber unit signal with the chip code sequence signal transmitted by the base station to determine the location of the subscriber unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a simplified, prior art CDMA system.

FIG. 2 is an illustration of a prior art CDMA system.

FIG. 3 is a block diagram of major components within a prior art CDMA system.

FIG. 4 is a block diagram of components within a prior art CDMA system.

FIG. 5 is an illustration of a global pilot signal and an assigned pilot signal being communicated between a base station and a subscriber unit.

FIG. 6 is a block diagram of a first embodiment of the present invention using at least three base stations.

FIG. 7 is an illustration of locating a subscriber unit using the first embodiment of the present invention with at least three base stations.

FIG. 8 is a block diagram of a second embodiment of the present invention showing components used in a subscriber unit.

FIG. 9 is an illustration of locating a subscriber unit using the second embodiment of the present invention with two base stations.

FIG. 10 is an illustration of locating a subscriber unit using the second embodiment of the present invention with more than two base stations.

FIG. 11 is a detailed illustration of the third embodiment of the present invention having a base station with multiple antennas.

FIG. 12 is an illustration of the third embodiment having a base station with multiple antennas.

FIG. 13 is a block diagram of components used in the third embodiment.

FIG. 14 is an illustration of multipath.

FIG. 15 is a graph of a typical impulse response of multipath components.

FIG. 16 is a block diagram of components within a fourth embodiment correcting for multipath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout.

Shown in FIG. 1 is a simplified CDMA communication system. A data signal with a given bandwidth is mixed with a spreading code generated by a pseudo random chip code sequence generator producing a digital spread spectrum signal. Upon reception, the data is reproduced after correlation with the same pseudo random chip code sequence used to transmit the data. Every other signal within the transmission bandwidth appears as noise to the signal being despread.

For timing synchronization with a receiver, an unmodulated pilot signal is required for every transmitter. The pilot signal allows respective receivers to synchronize with a given transmitter, allowing despreading of a traffic signal at the receiver.

In a typical CDMA system, base stations send global pilot signals to all subscriber units within their communicating range to synchronize transmissions in a forward direction. Additionally, in some CDMA systems, for example a B-CDMA™ system, each subscriber unit sends a unique assigned pilot signal to synchronize transmissions in a reverse direction.

FIG. 2 illustrates a CDMA communication system 30. The communication system 30 comprises a plurality of base stations 36 ₁, 36 ₂ . . . 36 _(n). Each base station 36 ₁, 36 ₂ . . . 36 _(n) is in wireless communication with a plurality of subscriber units 40 ₁, 40 ₂ . . . 40 _(n), which may be fixed or mobile. Each subscriber unit 40 ₁, 40 ₂ . . . 40 _(n) communicates with either the closest base station 36 ₁ or the base station 36 ₁ which provides the strongest communication signal. Each base station 36 ₁, 36 ₂ . . . 36 _(n) is in communication with other components within the communication system 30 as shown in FIG. 3.

A local exchange 32 is at the center of the communications system 30 and communicates with a plurality of network interface units (NIUs) 34 ₁, 34 ₂ . . . 34 _(n). Each NIU is in communication with a plurality of radio carrier stations (RCS) 38 ₁, 38 ₂ . . . 38 _(n) or base stations 36 ₁, 36 ₂ . . . 36 _(n). Each (RCS) 38 ₁, 38 ₂ . . . 38 _(n) or base station 36 ₁, 36 ₂ . . . 36 _(n) communicates with a plurality of subscriber units 40 ₁, 40 ₂ . . . 40 _(n) within its communicating range.

FIG. 4 depicts a block diagram of the pertinent parts of an existing spread spectrum CDMA communication system. Each independent base station 36 ₁, 36 ₂ . . . 36 _(n) generates a unique global pilot signal using a global pilot chip code generating means 42 ₁ and spread spectrum processing means 44 ₁. The global pilot chip code generating means 42 ₁ generates a unique pseudo random chip code sequence. The unique pseudo random chip code sequence is used to spread the resultant signals bandwidth such as to 15 MHZ as used in the B-CDMA™ air interface. The spread spectrum processing means modulates the global pilot chip code sequence up to a desired center frequency. The global pilot signal is transmitted to all subscriber units 40 ₁ by the base station's transmitter 46 ₁.

A receiver 48 ₁ at a subscriber unit 40 ₁ receives available signals from a plurality of base stations 36 ₁, 36 ₂ . . . 36 _(n). As shown in FIG. 5, the global pilot 50 ₁ travels from the base station 36 ₁ to the subscriber unit 40 ₁ and can be represented as: $\begin{matrix} {\tau_{1} = {\frac{d_{1}}{c}.}} & \text{Equation~~~(1)} \end{matrix}$

The time the signal travels from the base station 36 ₁ to the subscriber unit 40 ₁, τ₁, equals the distance between the base station 36 ₁ and subscriber unit 40 ₁, d₁, divided by the speed of light, c.

Referring back to FIG. 4, a global pilot chip code recovery means 54 ₁ within the subscriber unit 40 ₁ can receive global pilot chip code sequences from a plurality of base stations 36 ₁, 36 ₂ . . . 36 _(n). The subscriber unit 40 ₁ generates a replica of a global pilot chip code sequence and synchronizes the generated replica's timing with the received global pilot 50 ₁. The subscriber unit 40 ₁ also has a processor 82 ₁ to perform the many analysis functions of the subscriber unit 40 ₁.

The subscriber unit 40 ₁ generates an assigned pilot signal 52 ₁ using assigned pilot chip code generating means 56 ₁ and spread spectrum processing means 58 ₁. The assigned pilot chip code generating means 56 ₁ generates a pseudo random chip code sequence with its timing synchronized with the recovered global pilot chip code sequence. As a result, the assigned pilot chip code sequence is delayed by τ₁ with respect to the base station 36 ₁, 36 ₂ . . . 36 _(n). The spread spectrum processing means 58 ₁ generates the assigned pilot signal 52 ₁ by modulating the assigned pilot chip code sequence up to a desired center frequency. The assigned pilot signal 52 ₁ is transmitted to all base stations 36 ₁, 36 ₂ . . . 36 _(n) within range to receive the assigned pilot signal 52 ₁.

The base station 36 ₁ receives the assigned pilot signal 52 ₁ with the base station's receiver 62 ₁. The received assigned pilot 52 ₁ travels the same distance d₁ as the global pilot signal 50 ₁ as shown in FIG. 5. Accordingly, the received assigned pilot signal will be delayed by τ₁ with respect to the mobile unit 40 ₁ and by 2τ₁ with respect to the global pilot 50 ₁ generated at the base station 36 ₁.

Since the chip code sequence of the assigned pilot 52 ₁ received at the base station 36 ₁ will be delayed by 2τ₁ with respect to the chip code sequence of the global pilot signal 50 ₁ generated at the base station 36 ₁, the round trip propagation delay, 2τ₁, can be determined by comparing the timing of the two chip code sequences. Using the round trip propagation delay, 2τ₁, the distance d₁ between the base station 36 ₁ and subscriber unit 40 ₁ can be determined by: $\begin{matrix} {d_{1} = {c \cdot {\frac{2\tau_{1}}{2}.}}} & \text{Equation~~~(2)} \end{matrix}$

If a spreading sequence having a chipping rate of at least 80 ns is used and the communication system has the ability to track {fraction (1/16)}th of a chip, the distance d₁ can be measured to within 2 meters.

FIG. 6 is a block diagram of a first embodiment of the present invention. No additional hardware is required in the subscriber unit 40 ₁. The only changes are implemented by software within the subscriber unit's processor 82 ₁ and the processors 66 ₁, 66 ₂ . . . 66 _(n), 68, 70 ₁, 70 ₂ . . . 70 _(n) located within the base station 36 ₁, NIU 34 ₁ or Local Exchange 32 ₁, Precincts 74 ₁, 74 ₂ . . . 74 _(n) and Ambulance Dispatcher 76.

The subscriber unit 40 ₁ is sent a signal by a base station 36 ₁ indicating that a 911 call was initiated and to begin the subscriber location protocol. Upon receipt, the subscriber unit 40 ₁ will sequentially synchronize its transmission chip code sequence to at least three base stations' chip code sequences. To allow reception by the base stations 36 ₂, 36 ₃ . . . 36 _(n) outside of the subscriber unit's normal communicating range, these transmissions will be sent at a higher than normal power level temporarily over-riding any adaptive power control algorithms.

A processor 66 ₁ within each base station 36 ₁, 36 ₂ . . . 36 _(n) is coupled to the assigned pilot chip code recovery means 64 ₁ and the global pilot chip code generator 42 ₁. The processor 66 ₁ compares the two chip code sequences to determine the round trip propagation delay τ₁, τ₂ . . . τ_(n) and the respective distance d₁, d₂ . . . d_(n) between the subscriber unit 40 ₁ and the respective base station 36 ₁, 36 ₂ . . . 36 _(n).

Within either a NIU 34 ₁ or the local exchange 32, a processor 68 receives the distances d₁, d₂ . . . d_(n) from the processors 66 ₁, 66 ₂ . . . 66 _(n) within all the base stations 36 ₁, 36 ₂ . . . . 36 _(n). The processor 68 uses the distances d₁, d₂ . . . d_(n) to determine the location of the subscriber unit 40 ₁ as follows.

By using the known longitude and latitude from three base stations 36 ₁, 36 ₂, 36 ₃ and distances d₁, d₂, d₃, the location of the subscriber unit 40 ₁ is determined. As shown in FIG. 7 by using the three distances d₁, d₂, d₃, three circles 78 ₁, 78 ₂, 78 ₃ with radiae 80 ₁, 80 ₂, 80 ₃ are constructed. Each circle 78 ₁, 78 ₂, 78 ₃ is centered around a respective base station 36 ₁, 36 ₂, 36 ₃. The intersection of the three circles 78 ₁, 78 ₂, 78 ₃ is at the location of the subscriber unit 40 ₁.

Using the Cartesian coordinates, the longitude and latitude corresponding with each base station 36 ₁, 36 ₂ . . . 36 _(n) is represented as X_(n), Y_(n), where X_(n) is the longitude and Y_(n) is the latitude. If X, Y represents the location of the subscriber unit 40 ₁, using the distance formula the following equations result:

(X ₁ −X)²+(Y ₁ −Y)² =d ₁ ²  Equation (3)

(X ₂ −X)²+(Y ₂ −Y)² =d ₂ ²  Equation (4)

(X ₃ −X)²+(Y ₃ −Y)² =d ₃ ²  Equation (5)

In practice due to small errors in calculating the distances d₁, d₂, d₃, Equations 3, 4 and 5 cannot be solved using conventional algebra. To compensate for the errors, a maximum likelihood estimation is used to determine the location and are well known to those skilled in the art. For increased accuracy, additional base stations 36 ₄, 36 ₅ . . . 36 _(n) can be used to calculate additional distances for inclusion in the estimation analysis.

The subscriber unit's location is sent through the communication system 30 to at least one precinct 74 ₁, 74 ₂ . . . 74 _(n) and an ambulance dispatcher 76. A processor 70 ₁ within each precinct 74 ₁, 74 ₂ . . . 74 _(n) and the ambulance dispatcher 76 receives the location of all 911 calls originating in the system and displays the location on a conventional computer monitor 72 ₁. The display comprises a listing of all 911 calls and addresses on a geographic map.

An alternate approach reduces the number of processors by transmitting raw data through the communication system 30 and processing the raw data at a single site.

FIG. 8 is a second embodiment of a location system. At least two base stations 36 ₁, 36 ₂ . . . 36 _(n) have their internal timing synchronized with each other and transmit their respective global pilot signals 52 ₁, 52 ₂ . . . 52 _(n) with time synchronized chip code sequences. The subscriber unit. 40 ₁ receives the global pilots 52 ₁, 52 ₂ . . . 52 _(n). However, the received global pilots 52 ₁, 52 ₂ . . . 52 _(n) are not synchronized. The global pilot 52 ₁ from a first base station 36 ₁ will travel distance d₁ and is delayed by τ₁. The global pilot 52 ₂ from a second base station 36 ₂ travels distance d₂ and is delayed by τ₂. The subscriber unit 40 ₁ recovers each base station's global pilot chip code sequence with its global pilot chip code recovery means 54 ₁. A processor 82 ₁ within the subscriber unit 40 ₁ is coupled to each global pilot chip code recovery means 54 ₁, 52 ₂ . . . 54 _(n). The processor 82 ₁ compares the chip code sequences of each pair of pilot chip code sequences and calculates the time differences Δt₁, Δt₂ . . . Δt_(n) between the sequences as follows.

Within the subscriber unit 40 ₁, the chip code sequences used by each base station 36 ₁, 36 ₂ . . . 36 _(n) are stored. After synchronizing with the first base station's pilot 36 ₁, the processor 82 ₁ will store where within the sequence synchronization was obtained. This process is repeated for the other base stations 36 ₂, 36 ₃ . . . 36 _(n). The synchronization process can be done sequentially (synchronizing to the first base station's chip code sequence then the second, etc.) or in parallel (synchronizing to all base stations at the same time).

By using the relative time difference between τ₁, τ₂, . . . τ_(n) each base station's chip code sequence and knowing that each base station's pilot was sent at the same time, with two base stations the time differences are calculated as follows:

Δt ₁=τ₂−τ₁  Equation (6)

Δt ₂=τ₃−τ₂  Equation (7)

The time differences Δt₁, Δt₂ . . . Δt_(n) are transmitted to at least one of the base stations 36 ₁.

At least one base station 36 ₁ recovers the time difference data from the received signals using time difference recovery means 84 ₁. The time difference data is sent with the distance data d₁ through the communications system to a processor 68. The processor 68 determines the location of the subscriber unit 40 ₁ using the time difference data Δt₁, Δt₂ . . . Δt_(n) and the distance data d₁, d₂ . . . d_(n) as follows.

Using information from only two base stations 36 ₁, 36 ₂ as shown in FIG. 9, the processor uses distances d₁, d₂ to create two circles 78 ₁, 78 ₂. Using the time difference, Δt₁, a hyperbola 86 ₁ can be constructed as follows.

All the points along the hyperbola 86 ₁ receive the global pilot signals 52 ₁, 52 ₂ from the synchronized base stations 36 ₁, 36 ₂ with the same time difference, Δt₁. The time difference Δt₁ can be converted to a distance difference Δd₁ by substituting Δt₁ for t₁ and Δd₁ for d₁ in Equation 1. Using the distance formula and X,Y as the location of the subscriber unit 40 ₁, the following equation results:

Δd ₁={square root over ((X ₁ −X)²+(Y ₁ −Y)²)}−{square root over ((X ₂ −X)²+(Y ₂ −Y)²)}  Equation (8)

By using Equation 8 with Equations 3 and 4 in a maximum likelihood estimation, the location of the subscriber unit 40 ₁ can be determined. The subscriber unit's location is subsequently sent to the nearest police precinct 74 ₁, 74 ₂ . . . 74 _(n) and ambulance dispatcher 76 in the cellular area.

For improved accuracy, additional base stations 36 ₁, 36 ₂ . . . 36 _(n) are used. FIG. 10 shows the invention used with three base stations 36 ₁, 36 ₂, 36 ₃. The distances d₁, d₂, d₃ are used to create three circles 78 ₁, 78 ₂, 78 ₃. Using time differences Δt₁, Δt₂, two intersecting hyperbolas 86 ₁, 86 ₂ are constructed. With maximum likelihood estimation, the subscriber units' location calculated with two hyperbolas 86 ₁, 86 ₂, and three circles 78 ₁, 78 ₂, 78 ₃ yields greater accuracy.

As shown in FIG. 8, the subscriber unit 40 ₁ is required to process each global pilot chip code sequence to determine the time differences Δt₁, Δt₂ . . . Δt_(n). An alternate approach removes the processing from the subscriber unit 40 ₁.

With reference to FIG. 6, the mobile unit 40 ₁ will synchronize the assigned pilot to one of the base station's global pilot chip code sequences, such as the nearest base station 36 ₁ with a delay of τ₁. The assigned pilot 50 ₁ is transmitted to all base stations 36 ₁, 36 ₂ . . . 36 _(n). The assigned pilot 50 ₁ will be received at each base station with a respective delay, τ₁+τ₁, τ₁+τ₂, τ₁+τ₃. Each base station 36 ₁, 36 ₂ . . . 36 _(n) will send the delayed chip code sequence along with the calculated distance to a processor 68 located in a NIU 34 ₁ or local exchange 32. The processor 68 will calculate the time differences Δt₁, Δt₂ . . . Δt_(n) by comparing the received assigned pilot chip code sequences. Since all received assigned pilot chip code sequences are delayed by τ₁, the τ₁ delay will cancel out of the resultant time differences Δt₁, Δt₂ . . . Δt_(n). Accordingly, the subscriber unit 40 ₁ can be located using hyperbolas 86 ₁, 86 ₂ as previously described.

Another embodiment shown in FIGS. 11, 12 and 13 uses a base station 36 ₁ with multiple antennas 88 ₁, 88 ₂ . . . 88 _(n). Two of the antennas 88 ₁, 88 ₂ lie along a centerline 92 at a known distance, l, apart as shown in FIG. 11. Both antennas 88 ₁, 88 ₂ receive the assigned pilot signal 90 ₁, 90 ₂ from the subscriber unit 40 ₁. However, the antenna 88 ₂ further away from the subscriber unit 40 ₁ receives the signal over a slightly longer distance d₁′ and with a slight delay with respect to the nearer antenna 88 ₁. This delay results in a carrier phase difference, φ, between the signals received at each antenna as shown on FIG. 13. A processor 66 using the received carrier phase difference and the chip code sequence recovered by each assigned pilot chip code recovery means 96 ₁, 96 ₂ . . . 96 _(n) can determine the location of the subscriber unit 40 ₁ as follows.

As shown in FIG. 12, the subscriber unit 40 ₁ is located at distance d₁ at angle α from the centerline 92 of the antennas 88 ₁, 88 ₂. As seen at the scale of FIG. 12 both received assigned pilot signals 90 ₁, 90 ₂ appear to be coincident. However, as shown in FIG. 11, the received assigned pilot signals 90 ₁, 90 ₂ are slightly separated. The received assigned pilot signal 90 ₁ returning to the first antenna 88 ₁ travels a distance d₁. The received assigned pilot signal 90 ₂ returning to the second antenna 88 ₂ travels a slightly longer distance d₁′. As shown in FIG. 11, the difference between the two distances d₁, d₁′ is a distance m.

Since the distances d₁, d₁′ between the antennas 88 ₁, 88 ₂ and the subscriber unit 40 ₁ are much larger than the distance l between the antennae 88 ₁, 88 ₂ both received assigned pilot signals 90 ₁, 90 ₂ follow approximately parallel paths. By constructing a right triangle using a point 94 which is distance d₁ from the subscriber unit 40 ₁ as shown in FIG. 11, the angle ∝ can be determined by the following geometric relationship:

∝=COS⁻¹(m/l).  Equation (9)

The distance m can be determined by using the carrier phase difference, φ, between the two received signals 90 ₁, 90 ₂ as follows: $\begin{matrix} {m = \frac{\varphi \cdot \lambda}{2\pi}} & \text{Equation~~~(10)} \end{matrix}$

The distance m equals the phase difference between the two signals, φ, in radians multiplied by the wavelength of the signal, λ, divided by 2π. The wavelength, λ, can be derived from the known frequency f of the assigned pilot signal as follows:

λ=c/f.  Equation (11)

The processor 68 also compares the chip code sequences of the global pilot generating means 42 ₁ with the recovered assigned pilot chip code sequence to determine the distance d₁ as shown in FIG. 6. Using both the angle ∝ and distance d₁, the processor 66 ₁ locates the subscriber unit 40 ₁ using simple geometry. There are many techniques well known to those skilled in the art to eliminate the ambiguity between locations above and below the antennas 88 ₁, 88 ₂. One such technique is using antennas employing sectorization. Subsequently, the subscriber unit's location is sent to the precincts 74 ₁, 74 ₂ . . . 74 _(n) and ambulance dispatcher 76. Additional antennas may be used to improve on the accuracy of the system.

An alternate embodiment uses more than one base station 36 ₁, 36 ₂ . . . 36 _(n). A processor 68 located within either a NIU 34 ₁ or the local exchange 32 collects distance and angle information from more than one base station 36 ₁, 36 ₂ . . . 36 ₁ as well as the time differences Δt₁, Δt₂ . . . Δt_(n), between the base stations 36 ₁, 36 ₂ . . . 36 _(n). Using the maximum likelihood estimation technique, the processor 68 determines a more accurate location of the subscriber unit 40 ₁.

A fourth embodiment corrects for multipath. FIG. 14 illustrates multipath. A signal such as a global pilot signal is transmitted from a base station 36 ₁. The signal follows a multitude of paths 98 ₁, 98 ₂ . . . 98 _(n) between the base station 36 ₁ and subscriber unit 40 ₁.

FIG. 15 is a graph showing the impulse response 100 of the received multipath components. Since each received multipath component travels a unique path, it arrives at a receiver with a propagation delay determined by the length of the path 98 ₁, 98 ₂ . . . 98 _(n). The impulse response 100 shows the collective signal magnitude of all the multipath components received at each propagation delay.

The previously described subscriber unit location techniques assumed the subscriber unit 40 ₁ synchronizes with the line of sight multipath component 98 ₁ traveling distance d₁. However, if the subscriber unit synchronizes with a non-line of sight multipath component 98 ₁, 98 ₂ . . . 98 _(n), the distance calculation will be in error due to the delay MD₁ as shown in FIG. 15.

FIG. 16 is a system correcting for errors resulting from multipath. The global pilot 50 ₁ is sent from the base station 36 ₁ to subscriber unit 40 ₁. The subscriber unit 40 ₁ collects all of the multipath components using a multipath receiver 102 ₁ such as disclosed in U.S. patent application Ser. No. 08/669,769, Lomp et al., incorporated here by reference. A processor 82 ₁ within the subscriber unit 40 ₁ analyzes the impulse response 100 of the received global pilot signal 50 ₁.

Since the line of sight multipath component 98 ₁ travels the shortest distance d₁, the first received component 98 ₁ is the line of sight component. If the line of sight component is not received, the first received component 98 ₁ will be the closest and, accordingly, the best available estimate for the line of sight component. The processor 82 ₁ compares the chip code sequence of the first received component 98 ₁ with the chip code sequence used to synchronize the assigned pilot chip code sequence. This comparison determines the delay due to multipath, MD₁. The multipath delay, MD₁, is transmitted to the base station 36 ₁.

A processor 66 ₁ and multipath receiver 104 ₁ within the base station 36 ₁ perform the same analysis on the received assigned pilot signal. As a result, the multipath delay, MD₂, of the assigned pilot signal is determined. Additionally, multipath delay recovery means 106 ₁ recovers the transmitted global pilot signal's multipath delay MD₁ for use by the processor 66 ₁. The processor 66 ₁ compares the generated global pilot chip code sequence to the recovered assigned pilot chip code sequence to determine the round trip propagation delay 2τ₁. To correct for multipath, the processor 66 ₁ subtracts both the global pilot signal's multipath delay MD₁ and the assigned pilot signals multipath delay MD₂ from the calculated round trip propagation delay, 2τ₁. The corrected round trip propagation delay is used to determine the subscriber unit's location in one of the techniques as previously described.

Although the invention has been described in part by making detailed reference to certain specific embodiments, such detail is intended to be instructive rather than restrictive. It will be appreciated by those skilled in the art that many variations may be made in the structure and mode of operation without departing from the scope of the invention as disclosed in the teachings herein. 

What is claimed is:
 1. A method for geographically locating a subscriber unit using a plurality of base stations in a wireless CDMA communication system, each base station transmitting a first spread spectrum signal having a first code, for each received first signal, the subscriber unit transmits a second spread spectrum signal having a second code to that received first signal's base station time synchronized with that received first signal, the method comprising: for each received first signal at the subscriber unit, analyzing an impulse response of multipath components of that received first signal to determine a first received component of that received first signal; at each base station, analyzing an impulse response of multipath components of that base station's received second signal to determine a first received component of that base station's received second signal; for each base station, determining a distance between that base station and the subscriber unit based on in part a timing difference between the second code of the received second signal and the first code of that base station's transmitted first signal and the determined first received component for that base station's received second signal; and determining the location of the subscriber unit based on in part the determined distances, a fixed location of each base station and a maximum likelihood estimation.
 2. The method of claim 1 wherein the base stations are time synchronized to each other; the subscriber unit determines a time difference of reception between the received first signals and transmits a representation of the time difference; and the maximum likelihood estimation uses the determined time difference to determine the subscriber unit location.
 3. The method of claim 1 wherein the base stations are time synchronized to each other; a time difference of reception between each base station's received second signal is determined; and the maximum likelihood estimation uses the determined time difference to determine the subscriber unit location.
 4. The method of claim 2 wherein using a formula for a hyperbola associated with each time difference and a formula for a circle associated with each determined distance in the maximum likelihood estimation.
 5. The method of claim 1 wherein the transmission of each second signal is time synchronized to the determined first received component of its received first signal.
 6. The method of claim 1 wherein the subscriber unit transmitting, for each base station, a representation of a time difference between the first received component and a synchronization time of the transmitted second signal of that base station.
 7. A wireless CDMA communication system for locating a subscriber unit using a plurality of base stations, each base station transmitting a first spread spectrum signal having a first code, for each received first signal, the subscriber unit transmits a second spread spectrum signal having a second code to that received first signal's base station time synchronized with that received first signal, the system comprising: the subscriber unit comprising means, for each received first signal, for analyzing an impulse response of multipath components of that received first signal to determine a first received component of that received first signal; each base station comprising means for analyzing an impulse response of multipath components of that base station's received second signal to determine a first received component of that base station's received second signal, means for determining a distance between that base station and the subscriber unit based on in part a timing difference between the second code of the received second signal and the first code of that base station's transmitted first signal and the determined first received component for that base station's received second signal; and means for determining the location of the subscriber unit based on in part the determined distances, a fixed location of each base station and a maximum likelihood estimation.
 8. The system of claim 7 wherein the base stations are time synchronized to each other; the subscriber unit having means for determining a time difference of reception between the received first signals; and means for transmitting a representation of the time difference; and the maximum likelihood estimation using the determined time difference to determine the subscriber unit location.
 9. The system of claim 7 wherein the base stations are time synchronized to each other; the system further comprising means for determining a time difference of reception between each base station's received second signal; and the maximum likelihood estimation using the determined time difference to determine the subscriber unit location.
 10. The system of claim 8 wherein using a formula for a hyperbola associated with each time difference and a formula for a circle associated with each determined distance in the maximum likelihood estimation.
 11. The system of claim 7 wherein the transmission of each second signal is time synchronized to the determined first received component of its received first signal.
 12. The system of claim 7 wherein the subscriber unit comprising means for transmitting for each base station, a representation of a time difference between the closest line of sight component and a synchronization time of the transmitted second signal of that base station. 